Solvated electron reaction-rate constants

Jhe discovery by radiation chemists of solvated electrons in a variety of solvents (5, 16, 20, 22, 23) has renewed interest in stable solutions of solvated electrons produced by dissolving active metals in ammonia, amines, ethers, etc. The use of pulsed radiolysis has permitted workers to study the kinetics of fast reactions of solvated electrons with rate constants up to the diffusion-controlled limit (21). The study of slow reactions frequently is made difficult because the necessarily low concentrations of electrons magnify the problems caused by impurities, while higher concentrations frequently introduce complicating second-order processes (9). The upper time limit in such studies is set by the reaction with the solvent itself. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

We should remember (1) that the activation energy of eh reactions is nearly constant at 3.5 0.5 Kcal/mole, although the rate of reaction varies by more than ten orders of magnitude and (2) that all eh reactions are exothermic. To some extent, other solvated electron reactions behave similarly. The theory of solvated electron reaction usually follows that of ETR in solution with some modifications. We will first describe these theories briefly. This will be followed by a critique by Hart and Anbar (1970), who favor a tunneling mechanism. Here we are only concerned with fe, the effect of diffusion having been eliminated by applying Eq. (6.18). Second, we only consider simple ETRs where no bonds are created or destroyed. However, the comparison of theory and experiment in this respect is appropriate, as one usually measures the rate of disappearance of es rather than the rate of formation of a product. 

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

The hydrated electron is characterized by its strong absorption at 720 nm (e = 1.9 x 104 dm3 mol-1 cm-1 (Hug 1981) the majority of the oscillator strength is derived from optical transitions from the equilibrated s state to the p-like excited state (cf. Kimura et al. 1994 Assel et al. 2000). The 720-nm absorption is used for the determination of its reaction rate constants by pulse radiolysis (for the dynamics of solvation see, e.g Silva et al. 1998 for its energetics see, e.g Zhan et al. 2003). IP only absorbs in the UV (Hug 1981), and rate constants have largely been determined by EPR (Neta et al. 1971 Neta and Schuler 1972 Mezyk and Bartels 1995) and competition techniques (for a compilation, see Buxton et al. 1988). In many aspects, H and eaq behave very similarly, which made their distinction and the identification of eaq" difficult (for early reviews, see Hart 1964 Eiben 1970 Hart and Anbar 1970), and final proof of the existence of the... 

Soon after the discovery of the absorption spectrum of the solvated electron in pulse radiolysis experiments (Chapter 2), the rates and mechanisms of its reaction with a wide variety of solutes was studied. Although it is a transient species, the solvated electron is a very important reducing agent. Indeed, its reduction potential is very negative the value of E°(H2O/e5 ) for the solvated electron in water is equal to -2.8 V with respect to the standard hydrogen electrode. The reaction rate constant and the probability of encounter with another species decide the so-called lifetime ofthe solvated electron, which therefore depends on the experimental conditions. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

The development of the theory of the rate of electrode reactions (i.e. formulation of a dependence between the rate constants A a and kc and the physical parameters of the system) for the general case is a difficult quantum-mechanical problem, even when adsorption does not occur. It would be necessary to consider the vibrational spectrum of the solvation shell and its vicinity and quantum-mechanical interactions between the reacting particles and the electron at various energy levels in the electrode. 

Due to these reactions, hydrogen peroxide is an intermediate product of radiolysis of aerated water. Rate constants of free radical reactions with dioxygen and hydrogen peroxide are collected in Table 3.19. For the characteristics of solvated electron and information about its reactions, see monographs [219-223],... 

Rate Constants of Solvated Electron, H, and HO Reactions with Dioxygen and Hydrogen Peroxide in Water at Room Temperature [223-225]... 

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